Synthetic water-soluble epihalohydrin resins, such as polyamidoamine-epichlorohydrin wet strength resins, are used, for example, in manufacturing paper. Procedures for making polyamidoamine-epihalohydrin resins are well known. In order to maximize useful functionality in the final product, these processes typically involve reacting aqueous polyamidoamine with an excess of epihalohydrin to completely convert amine groups in the polyamidoamine (PAA) to epihalohydrin adducts.
During the reaction halohydrin groups are added at the secondary amine groups of the polyamidoamine as shown in the following example, using epichlorohydrin as the epihalohydrin: ##STR1## where R.sub.2 NH represents a secondary amine group of the polyamidoamine.
After the epihalohydrin has been added and when heat evolution has subsided, the reaction mixture is heated to effect crosslinking and viscosity increase. During this reaction azetidinium ions are formed that impart wet strength to paper by forming a strong crosslinked network with the paper fibers.
The reaction of epihalohydrin with the polyamidoamine also produces nonpolymeric halohydrins that are environmentally objectionable. They include the epihalohydrin residuals 1,3-dihalo-2-propanol (1,3-DHP), 2,3-dihalo-1-propanol (2,3-DHP), 3-halo-1,2-propanediol (3-HPD), and 2-halo-1,3-propanediol (2-HPD). In addition, some epihalohydrin may remain.
It is known from U.S. Pat. Nos. 4,857,586, 4,975,499, and 4,929,309 and European publication 0 334 157 that chlorohydrin residues can be removed from PAA/epichlorohydrin polymers by converting the chlorohydrin residues to epoxide with an inorganic base and then decomposing the epoxide with a nonchlorinated acid to give the ester.
In the patents mentioned above, the chlorohydrin-removing steps are initiated after the increase in viscosity is substantially complete: ##STR2## In these patents, the product is acidified to its storage pH or the product is added to the papermaking machine as soon as the chlorohydrin-removing steps are completed. ##STR3##
U.S. Pat. No. 5,017,642 discloses that epichlorohydrin/polyamidoamine wet strength resins containing a very small amount of organic halogen can be prepared by adjusting the reaction temperature and the molar ratio of the polyamidoamine and epichlorohydrin. The reaction is continued until the total amount of unreacted epichlorohydrin and of the other organic halogen compounds does not change substantially.
It is also known from U.S. Pat. No. 5,019,606, German patent publication DE 41 14 657 and European Patent EP 0 512 423 that chlorohydrin residues can be removed by adding both inorganic bases and amines. The chlorohydrin-removing steps are initiated after viscosity increase has taken place. Adding base when the resin is at or near final viscosity has the disadvantage that the concentration of chloride ion is high, making epihalohydrin residue removal more difficult and requiring more base or more cycles of basification/acidification.
It is known from U.S. Pat. Nos. 3,954,680 and 4,575,527 that storage stability can be provided by adding amines to resins made by the reaction of epichlorohydrin with amines having no functionality other than hydrocarbon functionality. The amine additive is introduced during the final step of the preparation, and reduction in the amount of chlorinated hydrocarbons in the resin product is not disclosed.
It would be advantageous to have a process for preparing polyamidoamine/epihalohydrin wet strength resins that provides reduced levels of unwanted by-products such as 1,3-dihalo-2-propanols and 1-halo-2,3-propanediols while providing equal or greater wet strengthening properties than resins having a higher content of epihalohydrin residuals.